This invention relates to nuclear reactors and in particular to an apparatus for holding down fuel assemblies within the reactor core.
In pressurized water reactors the coolant flow rate and fuel assembly flow resistance are such that the hydraulic uplift force is of sufficient magnitude to cause the assemblies to jitter and even lift off the core support structure. Various approaches have been used to eliminate this detrimental movement.
One suggested solution involves the use of a lock down device which attaches the lower end of the fuel assemblies to the core support structure. While this device will function properly it does introduce mechanical complexity since the device must not only lock and unlock remotely but it must release reliably after a year of operation in the reactor environment.
Another approach has been to use springs located above each fuel assembly which bear against an upper alignment plate, thereby urging the fuel assemblies down. As reactors have been designed with increasingly large hydraulic uplift forces the spring force and the springs have become very large. Any component at this location limits the ability to obtain a desirable flow pattern and tends to increase the pressure drop of the coolant.
In the design of nuclear reactors, a loss of coolant accident must be considered which involves a break of either the inlet or outlet line connected to the reactor vessel. In the event of a break of the outlet line the increased flow results in a substantial increase in the upward force on the fuel assemblies. A break of the inlet line on the other hand reverses the flow direction. Steam generated in the core remains behind as a steam pocket and forces the water backwardly through the core to the break at the inlet line. It would be desirable to maintain the water within the core while permitting the steam to escape through the break.